If someone runs the config snippet by @sinnlos23 but gets an error like
[...]/nixos-rebuild: line 174: /nix/store/zlws4p5azdj27g1nmri6k1c53bzv0psc-source/bin/nixos-rebuild: No such file or directory
there is a fix here: system.autoUpgrade failed to run.
Essentially, replace the flake source by "path:${rootPath}", which magically resolved this issue. Took me a while to find, so I am adding it here.